1. Field of the Invention
The present invention relates to a fuel cell.
2. Description of the Related Art
Fuel cells that generate electric power through an electrochemical reaction between hydrogen and oxygen have been attracting attention as an energy source. An example of such a fuel cell is a polymer electrolyte fuel cell that uses a polymer electrolyte membrane as an electrolyte membrane. In this polymer electrolyte fuel cell, in order to attain a desired power generation performance, it is necessary to regulate membrane moisture of the electrolyte membrane to retain appropriate proton conductivity. For this reason, in a polymer electrolyte fuel cell, it is necessary to humidify the electrolyte membrane during the power generation process. Moreover, in order to make the power generation performance uniform in the plane of a cell, it is also necessary to maintain uniform moisture across the plane of the electrolyte membrane.
A humidifier that humidifies oxidant gas supplied to the cathode has been used to humidify the electrolyte membrane of a polymer electrolyte fuel cell. The use of the humidifier has led to an increase in the size of the fuel cell system. In recent years, there has been proposed a technique in which water produced through the above-described electrochemical reaction during power generation is re-circulated within a fuel cell to humidify the oxidant gas and the electrolyte membrane without using a humidifier (see, for example, Japanese Patent Application Publication No. 2002-42844 (JP-A-2002-42844)). Hereinafter, the operation of humidifying oxidant gas, fuel gas, or an electrolyte membrane by re-circulating water produced during power generation within a fuel cell will be referred to as “self-humidification”.
The technique described in Japanese Patent Application Publication No. 2002-42844 (JP-A-2002-42844) enables self-humidification of oxidant gas. According to this technique, oxidant gas is humidified by water, and this humidified oxidant gas is used to humidify the electrolyte membrane. However, this technique does not provide sufficient humidification of the upstream side of the electrolyte membrane in the oxidant gas flow direction. A large variation in moisture thus occurs in the plane of the electrolyte membrane. Such a variation in moisture may occur not only on the oxidant gas side but also on the fuel gas side. This is because the water that is produced at the cathode may also be transmitted to the anode side through the electrolyte membrane.